Events: the "accept_mutex" directive is turned off by default.

Now it is believed that the accept mutex brings more harm than benefits.
Especially in various benchmarks it often results in situation where only
one worker grabs all connections.

src/event/ngx_event.c

diff --git a/src/event/ngx_event.c b/src/event/ngx_event.c
index c8ae5b2..2f3a098 100644
--- a/src/event/ngx_event.c
+++ b/src/event/ngx_event.c
@@ -1261,7 +1261,7 @@
     ngx_conf_init_ptr_value(ecf->name, event_module->name->data);
 
     ngx_conf_init_value(ecf->multi_accept, 0);
-    ngx_conf_init_value(ecf->accept_mutex, 1);
+    ngx_conf_init_value(ecf->accept_mutex, 0);
     ngx_conf_init_msec_value(ecf->accept_mutex_delay, 500);
 
     return NGX_CONF_OK;